Andrew Coppin wrote:
Is there any way that you can turn an arbitrary Haskell value into a string?
No, the only values of type a -> String are the constant functions and _|_.
I rephrase: There *is* a way to turn arbitrary values into strings. I know there is, because the GHCi debugger *does* it. The question is, does anybody know of an /easy/ way to do this?
No. GHCi does not always do this: Prelude Data.Ratio> let plus1 = (+1) Prelude Data.Ratio> plus1 <interactive>:1:0: No instance for (Show (a -> a)) arising from a use of `print' at <interactive>:1:0-4 Possible fix: add an instance declaration for (Show (a -> a)) In a stmt of a 'do' expression: print it Prelude Data.Ratio>
Basically, I'm writing a mutable container implementation. It can hold any type of data, but it would massively aid debugging if I could actually print out what's in it. On the other hand, I don't want to alter the entire program to have Show constraints everywhere just so I can print out some debug traces (and then alter everything back again afterwards once I'm done debugging).
This is not advisable, as you see.
Anybody know of a way to do this? (As it happens, the values I'm testing with are all Showable anyway, but the type checker doesn't know that...)
What is the problem with adding a function showMyContainer :: (Show a) => Container a -> String ? In this case you can show your container (for debugging purposes), but only if you have Showable elements in your container. Cheers, Jochem -- Jochem Berndsen | jochem@functor.nl | jochem@牛在田里.com